This application is related to the co-pending application of Thomas L. Blose, entitled TUBULAR CONNECTION HAVING A PARALLEL CHEVRON THREAD, application Ser. No. 06/565119 filed concurrently herewith.
The present invention relates generally to a thread form for a tubular connection of the type used in tubing casing and drill pipe, such as is used in gas and oil wells and, specifically to such a thread form designed with a chevron-shaped wedge thread.
With oil and gas wells being drilled to greater depths and in a greater variety of geographic locations than ever before, there exists a need for a tubular connection for pipe joints which will be capable of performing its sealing function under all conditions and under all levels of operating stress and strain. One object in designing such all purpose threads is to design a thread form that will have high torsional resistance without inducing axial or radial stresses into the tubular connection upon power make up of the joint. Tubular connections with high torsional resistance resist additional make up in the pipe joints when in service in the well bore, making it easier to break out the joints if this becomes necessary. By reducing axial or radial stresses in the threaded connection, a sounder connection is provided which is able to withstand a greater level of operating stress and strain.
In order to achieve the object of providing high torsional resistance in the threaded connection, the concept of a wedge thread was devised. The first wedge threads had low positive angle thread flanks on threads which were helically structured as a wedge. The theory was that when mating threads of helically wedged configuration were screwed together, as the wedging takes place, the torisional resistance of the connection greatly increases. Further advance or make up in the threaded connection could then only occur if the material of the threads themselves yielded due to the compressive or squeezing action of the load bearing faces of the threads. The wedge thread, if properly designed, can also help prevent inducing high axial or radial stresses during makeup of the connection by avoiding the necessity of "jack screwing" against a shoulder in the joint to absorb torque and limit make up. Unfortunately, machining technology has not until recent years been sufficiently advanced to accomplish the task of making a properly designed wedge thread on a practical basis for tubular manufacturing.
Another object in designing an all purpose thread form for well tubular connections is to provide a thread structure with a controlled clearance between mating crests and roots of the interengaged threads to prevent hydraulic pressure buildup caused by entrapped lubricant between the thread crests and roots. Entrapped lubricant has a tendency to lower breakout torque in a tubular connection, which is considered undesirable in well tubular connections.
Prior thread designs for well tubular connections have all been deficient in one or more of the above respects.